Light olefins (defined herein as ethylene, propylene, butenes and mixtures thereof) serve as feeds for the production of numerous chemicals and polymers. Light olefins traditionally are produced by petroleum cracking. Due to the escalating cost of crude petroleum, there are increasing efforts to develop light olefin production technologies based on alternative feedstocks. An important type of alternative feedstocks are oxygenates, such as alcohols, particularly methanol, dimethyl ether, dimethyl carbonate and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohols, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
Because light olefins are the most sought after products from the catalytic petroleum cracking and oxygenate conversion processes, a continuing need exists for new catalysts and/or new ways of making known catalysts to increase the yield of light olefin products and/or reduce the yield of unwanted products such as heavy hydrocarbons having molecular weights heavier than butane or low-valued by-products like methane.
Most catalysts that are used in the petroleum cracking and oxygenate conversion processes are molecular-sieve containing catalysts. A molecular sieve can be zeolitic--zeolites--or non-zeolitic. Typical examples of zeolitic molecular sieves are zeolite A, zeolite X, zeolite Y, ZSM-5, ZSM-34, erionite, chabazite, and others. A number of non-zeolitic molecular sieves, particularly silicoaluminophosphates (SAPO's) have been synthesized and investigated as catalysts for converting oxygenates or cracking heavy hydrocarbons to light olefins.
SAPO's have a three-dimensional microporous crystalline framework of PO.sub.2.sup.+, AlO.sub.2.sup.-, and SiO.sub.2 tetrahedral units. Because an aluminophosphate (AlPO.sub.4) framework is inherently neutral, the incorporation of silicon into the AlPO.sub.4 framework by substitution generates acid sites and acidity. Controlling the quantity and location of silicon atoms incorporated into an AlPO.sub.4 framework is important in determining the catalytic properties of a particular SAPO molecular sieve. Properly adjusted acid strength and acid site density are the keys to a good petroleum cracking or oxygenate conversion catalyst.
The catalytic properties of a SAPO catalyst also can be modified after the SAPO molecular sieve has been synthesized. This type of "post-synthesis" modification is accomplished by treating the molecular sieve with metallic, semi-metallic or non-metallic materials comprising nickel, cobalt, manganese, magnesium, barium, strontium, lanthanides, actinides, fluorine, chlorine, chelating agents, and others. The modifiers may or may not become part of the final composition of the modified catalyst.
SAPO's suitable for converting the oxygenates to light olefins include SAPO-16, SAPO-18, SAPO-34 and SAPO-44. These are small-pore molecular sieves with pore diameter smaller than about 5 Angstroms. Small pores are believed to favor light olefins production as a result of sieving effects. For the chabazite-like SAPO-34 and SAPO-44 molecular sieves, it may be possible to incorporate more silicon atoms into the tetrahedral positions of the framework to afford greater flexibility in adjusting their acidic properties.
A hydrothermal synthesis method for making SAPO-44 was described in U.S. Pat. No. 4,440,871. The starting materials were aqueous silica sol, aluminum isopropoxide, orthophosphoric acid, and an organic template, cyclohexylamine. The synthesis was performed at 200.degree. C. for 52 hours. The SAPO-44 was obtained as the "major phase" in the product, but the product was impure. It contained unidentified materials. A similar method base on the same starting materials and added hydrofluoric acid (HF) was reported by U. Lohse et al, in J. Chem. Soc. Faraday Trans. 91, 1155 (1995). In the presence of HF,, the reaction time was shortened to five hours at 200.degree. C. It is not clear how pure the SAPO-44 products were. Among the five products reported in the paper, at least one of them contained SAPO-35 impurity.
Because SAPO-44 can be used as a catalyst for the hydrocarbon cracking and oxygenate conversion processes, it is desirable to produce molecular sieve catalysts comprising SAPO-44 from cheaper starting materials and/or under less demanding reaction conditions. It is preferable to produce a molecular sieve product consisting essentially of SAPO-44. It is also preferable to avoid using highly corrosive reactants such as hydrofluoric acid (HF) in the molecular sieve synthesis reaction mixture.